The invention of the present application has application in circumstances where it becomes necessary during a drilling operation to drill through generally loose material or unstable ground conditions such as overburden, alluvial ground or the like in which it is difficult to establish bore hole stability during the drilling operation. The difficulties that these sorts of conditions create is that as the drill penetrates through the ground, the hole can backfill behind the drill bit and jam the drill in position which not only prevents the drill from being withdrawn from the hole but also can prevent further penetration of the drill. In addition, there are circumstances when it becomes necessary to drill through such loose ground conditions and into the solid base below these ground conditions in order to provide an anchor for structures which may be located above the ground.
In each of the above circumstances referred to above, it is appropriate to introduce a casing down the hole in order to stabilise the ground conditions and/or enable the location of an anchor in the bottom of the bore hole. It is preferable to introduce the casing into the bore hole during the progress of the drilling operation however, this can be difficult in loose ground conditions due to the possibility of the bore hole collapsing and jamming the casing.
In previous arrangements where a down hole hammer is used in association with a casing the exhausted fluid from the hammer is returned to the surface between the casing and the exterior of the hammer and drill string and as a result of the velocity of the fluid flow the cuttings generated by the action of the hammer are entrained in the fluid flow. In some ground conditions however there is a likelihood that the cuttings can build up in the annular space between the casing and the hammer and/or drill string which can create blockages and/or cause the hammer to become jammed within the casing to prevent the rotation of the drill string within the casing. A further difficulty can arise when the drill assembly passes through loose or unstable ground conditions or in the event that the drill bit encounters a void. Under these conditions the exhausted fluid will tend to escape into the loose ground or voids around the casing which results in a significant loss of pressure. This loss of pressure in turn reduces the entrainment capacity of the fluid flow which will cause the outtings to build up in the region of the drill bit or around the exterior of the casing. As a result the drill bit or casing can become jammed in the hole which will prevent further penetration of the drill assembly and possibly retraction of the drill assembly from the hole.
It is an object of this invention to provide a means whereby the down the hole percussive hammer can be utilised with the bore hole casing to penetrate through unstable ground conditions and in which the exhausted fluid and entrained outings can be conducted to the surface through the hammer and the drill string rather than through the bore hole and/or the casing.